In the oil and gas industry, a drilling fluid (“mud”) is used during drilling of a wellbore to facilitate the drilling process and to maintain a hydrostatic pressure in the wellbore greater than the pressure in the formations surrounding the wellbore. This drilling fluid penetrates into or invades the formations for varying radial depths (referred to generally as the invaded zones) depending upon the types of the formation and drilling fluid used. Wireline formation testing tools lowered into the mud of the wellbore are used to monitor formation pressures, collect formation fluid samples from the wellbore and to predict performance of reservoirs around the wellbore. These formation evaluation tools typically contain an elongated body having an elastomeric packer that is sealingly urged against the zone of interest in the wellbore. Fluid is collected and brought to the surface for analysis to determine the properties of the fluids and the conditions of the zones or formations from where the fluids have been collected. During this process, it is critical that only uncontaminated fluids are collected, and in the same condition in which they exist in the formation.
Formation evaluation tools typically collect formation fluid by transferring such fluids from a probe into a sample chamber. Prior art formation evaluation tools such as sequential formation testers and repeat formation testers used large collection chambers that varied in size from one to five gallons to collect samples. Samples were not pumped into the chamber, but were forced into the chamber by the hydrostatic pressure of the formation acting against the atmospheric pressure in the chamber. The problem with these chambers was that once opened at the formation zone, they would ingest not only the sample, but also surrounding mud, rocks and other contaminates. Current formation testing tools overcome this problem by first testing fluids from the desired formations or zones of interest to ensure that the fluid is substantially free of mud filtrates, and then collecting fluids by pumping formation fluid into one or more sample bottles associated with the tool.
Because of the great difference in pressure between the formation (hydrostatic) and the interior of the sample bottle (atmospheric), there is a possibility that the formation fluid pumped into the chamber will vaporize, or “flash,” due to a great decrease in pressure. In order to prevent or reduce the chances of the liquid vaporizing from a decrease in pressure, formation fluid is pumped into the chamber at a relatively slow rate. In addition, the tools are often equipped with restrictions to slow down the fluid flow rate into the chamber. Water cushions are also utilized to fill the chambers more uniformly. However, it is common for the collected single phase fluid to separate into a two phase sample containing vaporized gas. If the sample fluid pressure is reduced prior to arrival in the analysis lab, a lengthy procedure is required to recombine the sample back into a single phase as it was in situ. Additionally, asphaltenes are commonly present in the hydrocarbons and if the pressure in the chamber remains at a relatively low pressure, such asphaltenes tend to flocculate to form gel-type masses in the fluid. The flocculation process is substantially irreversible. Thus, it is desirable to withdraw and maintain the sample fluid at a pressure above the bubble point to maintain it in a single phase.
Additionally, the temperature difference between the surface elevation and the formation elevation can exceed several hundred degrees Fahrenheit. As the tool is retrieved, the chamber temperature drops, causing the pressure in the chamber to drop. This substantial pressure drop in the chamber can result in the pressure of the formation sample dropping below the bubble point, resulting in a multi-phase sample.
Attempts have been made to maintain the fluid sample in a single phase by applying a pressurized nitrogen charge against a sample piston located in the chamber. This forces the sample piston against the fluid sample to maintain its pressure at a sufficient level to prevent a phase change upon retrieval. However, this system is complex and requires the use of nitrogen at a pressure of over 20,000 psi. The danger and inconvenience of working with nitrogen at this extremely high pressure discourages its use.
The present invention addresses the above noted problems and provides a single phase collection apparatus in which collected formation fluid is maintained at a predetermined pressure above the bubble point to maintain the sample in a single phase. No water cushions are required to uniformly fill the chambers. The tool also automatically maintains the chamber pressure above the bubble point pressure during the entire sampling operation regardless of the change in the temperature surrounding the chamber.